With increase of emphasis on energy saving in recent years, high efficiency of a power conversion circuit is focused. The power conversion circuit converts a power of a form suitable for power generation and power transmission into a power of a form which can be used for power-consuming equipment easily. As the power conversion circuit, a switching power supply or an inverter is known, for example.
A device called as a power semiconductor device generally is used as a main device in a power conversion circuit. The power semiconductor device serves to control power supply to a load by a switching operation. Since a magnitude of loss in switching typically affects efficiency of a power conversion circuit significantly, research on high performance of a power semiconductor device is widely carried out. As a semiconductor material composing a power semiconductor device, silicon (Si) has been used for many years. However, the performance of a power semiconductor device is reaching a limit to which the performance is limited by the material properties of silicon.
A device which uses a new material such as silicon carbide (SiC) or a nitride semiconductor such as GaN in place of silicon has been developed for the purpose of exceeding the limit due to the material properties of silicon. The common features of such a new material are a wide band gap and large breakdown field strength, for example. Thus, in a case of employing SiC or GaN, it is possible to use a unipolar transistor having a fast switching speed even in a high voltage area in which a transistor other than a bipolar transistor can not be used when silicon is employed as a constituent material. As a result, it is possible to reduce loss which is produced in a switching operation of a power semiconductor device.
An energy saving effect is expected in a case of using such a new material in a diode as well as in a transistor. By the same reason as shown in the case of the transistor, it is possible to use a unipolar-type Schottky barrier diode (SBD) even in a high voltage area in which a diode other than a bipolar-type pn diode can not be used when silicon is employed as a constituent material. The SBD has a fast switching speed compared with the pn diode. Accordingly, it is possible to reduce switching loss and obtain a low-loss power conversion circuit.
A feature of the nitride semiconductor is that a two-dimensional electron gas (2DEG) with a high concentration and a high mobility is produced at a hetero interface when a hetero structure is formed by suitable material selection. A field effect transistor using the 2DEG as a channel is called as a hetero-structure field effect transistor (HFET). The HFET is focused as a device which has high voltage endurance and low on-resistance characteristics and can exhibit features of the nitride semiconductor to a maximum.
As a basic circuitry element constituting a power conversion circuit, a circuit called as a half bridge generally is known. The half bridge circuit is a circuit having a field effect transistor and a diode connected in reverse-parallel to the field effect transistor. In the half bridge circuit, the field effect transistor and the diode are connected in series. Various kinds of inverter circuits are realized by using a single piece of half bridge circuit or by using a plurality of half bridge circuits connected in parallel with each other.
The field effect transistor of the half bridge circuit performs a switching operation according to a signal which is input to a gate, and controls power supply from a power supply terminal to a load connected to a load connection terminal. The diode connected in reverse-parallel to the field effect transistor is generally called as a wheeling diode, and is provided to prevent breakdown of the field effect transistor by feedback power which is produced mainly in a case of using a coil load in a process of power control. Accordingly, a basic inverter circuit is usually provided with such a field effect transistor and a diode which compose a pair.
With regard to a nitride semiconductor, it is difficult to obtain a low-priced stand-alone substrate having a large diameter (4 inches or more) as that of silicon. For this reason, a substrate for the use of a semiconductor device of nitride is generally obtained by growing a nitride semiconductor on a sapphire substrate or a silicon substrate through hetero-epitaxial growth.
Since a substrate on which a crystalline nitride layer is formed may use a low-priced silicon substrate having a large diameter, it is possible to obtain the substrate at a relatively low price.
However, it is necessary to use an advanced and complicated technology to obtain a hetero-epitaxial crystal of high quality. Thus, even when such a substrate is used, an influence of increasing in an area which is occupied by the semiconductor device of nitride on a device cost is remarkable high as compared with a silicon device.
Accordingly, there is a need for a technology to integrate a field effect transistor and a diode to be connected in reverse-parallel to the field effect transistor with as little increase of area to be occupied by a device including the transistor and the diode as possible.